This invention relates to the purification of 1,1-dichloro-1-fluoroethane, also designated HCFC-141b, which has been of particular interest as a replacement for chlorofluorocarbons having similar physical properties, particularly HCFC-11 and -113. HCFC-141b may be prepared by reaction of vinylidene chloride or trichloroethane with HF. Such processes are disclosed, for example, in U.S. Pat. Nos. 2,894,044 and 3,833,676.
It is characteristic of such reactions that many by-products are formed, containing varying numbers of hydrogen, chlorine, and fluorine atoms on methane, ethane, and ethylene molecules. These by-products and the unreacted feed material may be separated by distillation where possible. Some compounds are relatively harmless since their presence does not greatly alter the physical properties for which HCFC-141b is useful. Vinylidene chloride has a boiling point close to that of HCFC-141b making it difficult to separate them by distillation. Since vinylidene chloride is toxic, it must be removed from HCFC-141b. After distillation of the crude product, vinylidene chloride will still be present in amounts from about 500 to 1,200 ppm (wt.). It should be reduced to below 500 ppm according to the specifications of the Panel for Advancement of Fluorocarbon Test (PAFTII). Preferably, the vinylidene chloride should be below about 200 wt. ppm.
Dichloroacetylene is another toxic impurity. It may be present in crude HCFC-141b in amounts of about 5 to 25 ppm (wt.) and should be removed to below 1 ppm to meet the specifications referred to above.
Various methods have been suggested for removing vinylidene chloride and dichloroacetylene from waste streams. For example, in U.S. Pat. No. 4,940,824 it is shown that vinylidene chloride can be removed from HCFC-141b using carbon molecular sieves and in U.S. Pat. No. 4,940,825 that dichloroacetylene also can be removed from HCFC-141b or from vinylidene chloride.
In. U.S. Pat. No. 4,948,479 Brooks et al. disclose the use of photochlorination to convert unsaturated carbon compounds, including vinylidene chloride from HCFC-141b. The patentees limit their process to photochlorination in the liquid phase and appear to infer that similar vapor phase photochlorination was known. However, while it is true that photochlorination of vinylidene chloride has been studied by Ayscough et al., Trans. Faraday Soc. 62(7) 1838-45(1966), and photochlorination of other unsaturated and chlorinated hydrocarbons have been reported, as Brooks et al. have discussed, it appears that the vapor phase photochlorination of unsaturated compounds, and particularly vinylidene chloride has not been disclosed to be an effective method of removing vinylidene chloride from HCFC-141b. It should be noted that the vapor phase photochlorination studies of Ayscough et al. involved below atmospheric pressure conditions with only vinylidene chloride and chlorine present, except where inert gases were introduced. Generally, the partial pressure of vinylidene chloride was higher than in the present process and since no other reactive species were present (e.g. HCFC-141b) one would not conclude from the results achieved in such a simplified model system that when large amounts of reactive species were present that vinylidene chloride could be selectively photochlorinated.
If possible, the use of vapor phase chlorination would present advantages, particularly since the purification steps are simplified, many of which can be carried out in the vapor phase, thus eliminating the need to condense and distill HCFC-141b before chlorination and then to vaporize it again for removal of acid gases. However, one skilled in the art might conclude from the available information that reducing the small amount of vinylidene chloride present in HCFC-141b by photochlorination could be unprofitable since extremely high selectivity would be required to chlorinate vinylidene chloride at low concentrations in HCFC-141b without chlorinating the HCFC-141b as well and vapor phase free radical chlorination would not be expected to be very selective. However, we have now found that vapor phase photochlorination is a suitable procedure for achieving low levels of vinylidene chloride and other unsaturated compounds when combined with other steps in a process for purification of crude HCFC-141b.